The present invention is broadly concerned with lean NOx abatement in gaseous streams such as the exhaust of lean-burn internal combustion engines, e.g., diesel engines. More specifically, the present invention is concerned with a novel method and an apparatus for carrying out the method, comprising use of a trap member having thereon an intimate combination of a sorbent material and an oxidation catalyst.
The art has devoted some effort to developing catalysts specifically designed to abate NOx in lean exhaust streams. One such effort is described at pages 45-48 of Environmental Catalysis For A Better World And Life, Proceedings of the 1st World Congress at Pisa, Italy, May 1-5, 1995, published by the Societa Chimica Italian of Rome, Italy, in an article entitled xe2x80x9cThe New Concept 3-Way Catalyst For Automotive Lean-Burn Engine Storage and Reduction Catalystxe2x80x9d, by Takahashi et al (below referred to as xe2x80x9cthe Takahashi et al paperxe2x80x9d). This article discloses the preparation of catalysts by impregnating precious metals, mainly platinum, and various alkaline and alkaline earth metal oxides, mainly barium oxide and rare earth metal oxides, on supports, mainly alumina. At page 47 of the article, there is, shown both graphically in FIG. 5 and discussed in paragraph 3.1.1., xe2x80x9cNOx Storage Mechanismxe2x80x9d, the concept of employing NOx storage compounds and three-way conversion (xe2x80x9cTWCxe2x80x9d) catalytic components dispersed on a common support material. TWC catalysts are well known in the art and are capable of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. However, TWC catalysts require stoichiometric or rich gas conditions in order to convert NOx desorbed from the sorbent material.
SAE paper 950809 published by the Society of Automotive Engineers, Inc., Warrendale, Pa., and entitled Development of New Concept Three-Way Catalyst for Automotive Lean-Burn Engines, by Naoto Miyoshi et al, was delivered at the International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995. This paper, referred to below as xe2x80x9cthe SAE paperxe2x80x9d, deals with gaseous streams at rich or stoichiometric conditions and discloses at page 123 a correlation between basicity of the elements used as NOx storage material and NOx storage capacity. Also discussed is NOx adsorption by complex oxides such as YBa2Cu3Oy and CuO-BaO. The SAE paper also references xe2x80x9cUptake of NO gas by YBa2Cu3Oyxe2x80x9d by K. Tabata et al, Chem. Lett., 1988, pp. 799-802 and xe2x80x9cNO removal by adsorption into BaO-CuO binary oxidesxe2x80x9d by M. Machida et al, J. Chem. Soc. Chem. Commum. 1990, pp. 1165-1166.
U.S. Pat. No. 5,202,300, xe2x80x9cCatalyst For Purification of Exhaust Gasxe2x80x9d, issued on Apr. 13, 1993, to M. Funabiki et al, discloses a catalyst composition comprising a refractory support having deposited thereon an active layer containing a palladium and rhodium catalytic metal component dispersed on alumina, a cerium compound, a strontium compound and a zirconium compound.
Generally, the present invention provides a method for treating a lean, NOx-containing gaseous stream, the temperature of which fluctuates and may remain for periods of time within a temperature range which is unsuitable for catalytic abatement of NOx. The method provides for using a trap member containing an intimate combination of a NOx sorbent and an oxidation catalyst and sorbing NOx onto the trap member during those selected periods of time, e.g., when the temperature of the gaseous stream is not suited for catalytic lean NOx abatement. During other periods of time, e.g., when the temperature of the gaseous stream being treated is suitable for catalytic lean NOx abatement, the method provides for introducing a combustible component, e.g., hydrocarbons, into the trap member in amounts which are limited in order to maintain the bulk or overall composition of the gaseous stream at lean conditions, and oxidizing the combustible component thereon to thermally desorb the NOx from the trap member without significantly raising the bulk temperature of the gaseous stream. The resulting NOx-enriched gaseous stream can be more efficiently treated to abate NOx for two reasons. One is because of its increased NOx concentration relative to the source gaseous stream and the other is because its bulk temperature need not be heated to NOx thermal desorption temperatures which are higher than the temperature window for most or many lean NOx abatement methods. The source of the gaseous stream, e.g., a diesel engine or a lean-burn gasoline engine, need not be subjected to periods of rich or stoichiometric operation to desorb the NOx and reduce it over a TWC catalyst, but may operate continuously under lean conditions. The amount of combustible component, e.g., hydrocarbon, introduced to effectuate the thermal desorption need not be so much as to convert the lean gaseous stream to a hydrocarbon (fuel)-enriched gaseous stream.
In accordance with the present invention there is provided a method for the reduction of NOx in a lean NOx-containing gaseous stream, e.g., the exhaust of a diesel engine. The method comprises the following steps. The lean gaseous stream is passed through a catalyzed NOx trap member comprising an intimate combination of a catalyst material effective for promoting the oxidation of a combustible component in the gaseous stream, and a NOx sorbent material. The gaseous stream is passed through the trap member within a sorbing temperature range to sorb NOx in the trap member whereby to establish a trapping period and provide a NOx-depleted gaseous stream exiting the trap member during the trapping period. The gaseous stream exiting the trap member is then passed through a lean NOx reduction catalyst member. The combustible component is introduced into the gaseous stream at a location upstream of the trap member under conditions to combust the combustible component on the trap member, the combustible component being introduced in an amount which is limited in order to maintain the bulk composition of the gaseous stream lean but which is sufficient to heat at least a portion of the trap member to a release temperature sufficient to release trapped NOx from the trap member into the gaseous stream exiting the trap member, thence through the catalyst member. This serves to establish a desorbing period. Lean NOx-reduction conditions are maintained in the catalyst member at least during the desorbing period, e.g., by supplying a NOx reductant to the catalyst member, and the establishment of the trapping period and the desorbing period is alternated.
One aspect of the present invention comprises introducing a NOx reductant into the lean gas stream between the trap member and the catalyst member.
Another aspect of the present invention provides for maintaining a molar ratio of reductant (on a C1 basis) to NOx of from about 0.5 to 8, preferably from 1 to 4, in the gaseous stream entering the catalyst member.
Other aspects of the present invention include one or more of the following features, alone or in combination: introducing the combustible component upstream of the trap member in an amount sufficient to supply from the combustible component at least part of the NOx reductant, and maintaining lean NOx reduction conditions in the catalyst member even while passing the NOx-depleted gaseous stream therethrough.
The present invention also provides for carrying out the method wherein the NOx sorbent material comprises at least one basic oxygenated compound and the catalytic metal component of the catalyst material intimately combined with the sorbent material may comprise one or more catalytic metal components, e.g., platinum group metal components, and preferably platinum. As for the catalyst member positioned downstream of the trap member, i.e., to receive the gaseous stream discharged therefrom, the catalyst member comprises a lean NOx abatement catalyst selected from the group consisting of a platinum catalytic component and a metal-promoted zeolite. In one embodiment of the invention, the NOx-sorbent material is selected from the group consisting of one or more of metal oxides, metal carbonates, metal hydroxides and mixed metal oxides and the catalyst member comprises a lean NOx abatement catalyst selected from the group consisting of a platinum catalytic metal component dispersed on a suitable support material and a copper or platinum catalytic component supported on a zeolite (molecular sieve), e.g., ZSM-5. The sorbent material may be present in bulk form, i.e., as fine, solid particles, or it may be dispersed on a suitable support material, i.e., fine solid particles of an oxide such as gamma-alumina. The terms xe2x80x9cbulkxe2x80x9d and xe2x80x9cdispersed onxe2x80x9d as used to describe a material are defined below.
Another aspect of the invention provides for the sorbent material to be segregated from the catalytic metal component of the oxidation catalyst, at least to the extent that (a) when the sorbent material is dispersed on a support material, the catalytic metal component and the sorbent material are not dispersed on the same increment of support material, and (b) when the sorbent material is in bulk form, the catalytic metal component is not incorporated into the bulk sorbent material.
Still other aspects of the present invention provide for maintaining the gaseous stream exiting the trap member within a temperature range below that of the release temperature range, and preferably within an operating temperature range effective for lean NOx abatement at least during the desorption periods.
Yet another aspect of the present invention provides an apparatus for the reduction of NOx in a lean, NOx-containing gaseous stream. The apparatus comprises the following components: an inlet conduit which is dimensioned and configured for connection in flow communication to a source of a lean gaseous stream containing NOx; a catalyzed NOx trap member having a trap inlet and a trap outlet, the trap inlet being connected in flow communication to the inlet conduit, the trap member comprising an intimate combination catalyst material effective for promoting the oxidation of hydrocarbons in the gaseous stream and a NOx sorbent material; a NOx catalyst member having a catalyst inlet and a catalyst outlet; a connector conduit connecting the trap outlet to the catalyst inlet in flow communication; combustible component inlet means connected in flow communication to the inlet conduit; and control means associated with the inlet conduit to permit intermittent injection of a combustible component into the inlet conduit.
The apparatus may further include a reductant inlet conduit having one end dimensioned and configured for connection to a source of a NOx reductant and an opposite end connected in flow communication to the connector conduit. The NOx trap member and NOx abatement catalyst may be those described above. Control means may also be associated with the reductant conduit to permit controlled injection of a NOx reductant into the connector conduit for introduction into the NOx catalyst member.
Another aspect of the present invention provides for the sorbent material to be segregated to a degree from the catalytic metal component in the intimate combination of the two. Such segregation is provided at least to the extent that (a) when the sorbent material is dispersed on a refractory support material, the catalytic metal component and the sorbent material are not dispersed on the same increment of refractory support material, and (b) when the sorbent material is in bulk form, the catalytic metal component is not, incorporated, e.g., impregnated into the bulk sorbent material.
As used herein and in the claims, the following terms, whether used in singular or plural form, have the indicated meanings.
Reference to a xe2x80x9ccatalytic metal componentxe2x80x9d, or a xe2x80x9cplatinum metal componentxe2x80x9d, or to a metal or metals comprising the same, means a catalytically effective form of the metal or metals, whether the metal or metals are present as the element, an alloy or a compound.
The term xe2x80x9cplatinum group metalsxe2x80x9d means and includes platinum, rhodium, palladium, ruthenium, iridium and osmium.
The term xe2x80x9cwashcoatxe2x80x9d has its usual meaning in the art of a thin, adherent coating of a catalytic or other material applied to a refractory carrier material such as a honeycomb-type carrier member, and which is porous to permit the passage therethrough of the gas stream being treated.
A xe2x80x9cbulkxe2x80x9d component means a component which is present as fine, solid particles, i.e., as a powder. The particles are usually quite small in size, on the order of, e.g., at least ninety percent of the particles being from 5 to 15 microns in diameter. For example, a bulk sorbent material is a powder of strontium oxide.
A component is xe2x80x9cdispersedxe2x80x9d onto a bulk support material by immersing the bulk support material into a solution or other liquid suspension of the component or a precursor thereof. For example, the sorbent strontium oxide may be dispersed onto an alumina support material by soaking bulk alumina in a solution of strontium nitrate (a precursor of strontia), drying the soaked alumina particles and heating them, e.g., in air at a temperature of 450xc2x0 C. to 750xc2x0 C. (calcining) to convert the strontium nitrate to strontium oxide dispersed on the alumina support materials.
The term xe2x80x9cgaseous streamxe2x80x9d means a stream of gaseous constituents, such as the exhaust of an internal combustion engine, which may contain entrained non-gaseous components such as liquid droplets, solid particulates and the like.
The abbreviation xe2x80x9cTOSxe2x80x9d means time on stream and the abbreviation xe2x80x9cHCxe2x80x9d means hydrocarbon.
The term xe2x80x9cleanxe2x80x9d used with reference to a mode of treatment (such as xe2x80x9clean NOx abatementxe2x80x9d) or a gaseous stream, means that the gaseous stream being subjected to the treatment, or the gaseous stream referred to, contains more oxygen than the stoichiometric amount of oxygen needed to oxidize the entire reductants content, e.g., HC, CO and H2, of the gaseous stream.
The term xe2x80x9cmixed metal oxidexe2x80x9d means bi-metallic or multi-metallic oxygen compounds, such as Ba2SrWO6, which are true compounds, and is not intended to embrace mere mixtures of two or more individual metal oxides such as a mixture of SrO and BaO.
The use of weight per volume units such as xe2x80x9cg/in3xe2x80x9d, or xe2x80x9cg/ft3xe2x80x9d describes the weight of a component per volume of catalyst or trap member including the volume thereof attributed to void spaces such as gas flow passages.
The term xe2x80x9ceffective operating temperature rangexe2x80x9d of a lean NOx abatement catalyst means the temperature range in which the catalyst is effective to promote reduction of at least about ten percent of the NOx contained in the gaseous stream fed to the catalyst when an adequate supply of reductant is present.
The term xe2x80x9cincrementxe2x80x9d of a support material means a batch or portion of support material; references to components not being disposed on the same xe2x80x9cincrementxe2x80x9d of support material simply means that the components are not dispersed on the same particles, but each component is dispersed on its own batch of particles.